Automotive systems are increasing the employment of sensing devices, such as those used in accelerometers. The sensing devices are now found in various automotive systems including vehicle navigation systems, chassis control systems having active suspensions and active rear steering, and an anti-slip regulation system, such as anti-lock braking. One such sensing device is used in an accelerometer described in U.S. patent application Ser. No. 08/038,422, filed Mar. 29, 1993, having Attorney Docket No. G-9949, assigned to the same assignee as the present invention, and which is herein incorporated by reference.
One of the more popular sensing devices is a piezoresistor that finds use in both accelerometers and pressure sensors. For an accelerometer, the piezoresistor, or an array of such elements, may be placed on or in a beam that supports a sensing mass whose displacement, relative to a fixed frame, is indicative of acceleration. The piezoresistors are arranged so as to be subjected to stresses whereby their resistance varies in response to the deflection of the beam which, in turn, is indicative of the displacement to which the sensing mass is being subjected. The piezoresistors, as well as other type sensing element, may be deposited directly onto a substrate preferably formed of alumina which, in turn, may serve as, e.g., a surface of the beam that supports the sensing mass. The sensor assemblies, such as an accelerometer, that incorporate the stress responsive piezoresistors create sites on their substrate which undergo high strain when the sensors are stressed. These highly strained sensor assemblies encounter two intertwined problems. The first of which is that the highly stressed areas, created during intended normal usage, inevitably result in a tendency to break when subjected to high forces. These high forces commonly occur when a sensor assembly is inadvertently dropped causing breakage thereof. The second problem is that when the sensor assembly is highly stressed, in particular the substrate where the piezoresistors are deposited, the material forming the piezoresistance undergoes a change of resistance which is only on the order of 0.1% of its total resistance, thereby, limiting the sensitivity of the device. The overall effect of these two problems is that a sensor assembly using piezoresistors acts as a fragile device that may easily be harmed when placed in an automotive system.